Chordz is a free VST plugin that allows you to trigger full chords by playing single notes. Each trigger note is associated with a separate chord. Each chord may consist of any number of notes. Play complicated chord progressions with one finger, with a MIDI keyboard or a drum-pad, or trigger chords from single notes added in your DAW’s piano roll.
RandARP is a VST2 plugin arpeggiator with various randomization features.
Chordit is a VST2 plugin that allows you to trigger chords by playing a single note.